Conventional ammunition projectiles such as used both in small caliber and large caliber weapons typically comprises a solid mass with a rounded nose or ogive portion, a generally cylindrical body and an aft or tail portion terminating abruptly in a flat surface normal to the longitudinal center axis of the cylindrical body. Since projectiles used in weaponry usually leave the gun tube or barrel at supersonic velocity, a relatively blunt nose produces very high drag force and the familiar parabolic shock wave. The blunt tail section results in considerable turbulene aft of the projectile which translates into further drag force from conversion of energy from the projectile to the surrounding mass of air.
Numerous design efforts have been used to reduce total drag on projectiles and thereby increases their impact force. Foremost among these efforts is the use of a hollow center passage thru the projectile such as to form a tubular shape. Some examples of the prior art demonstrating this design approach are shown in FIGS. 1 through 6.
An English inventor named Whitworth designed projectile 16 shown in FIG. 1 about 1857. Hole 17 is provided thru an elongate body 18 having a polygonal cross-section configuration resulting in multiple external surfaces as shown. There is little historical evidence that the design approach was ever adopted or actually used in warfare, from which it appears that it drew very little interest among those working in the ballistic art. The projectile suggested in FIG. 2 and variations thereof were used experimentally in 1893 by someone named Hebler of Switzerland. Projectile 19 was a conventional projectile with a longitudinal passage 20 provided through the axial center thereof. The experiments became known as the Krnka-Hebler experiments. Interest in the United States was evidenced in 1894 when experiments were conducted at Frankford Arsenal, Philadelphia, Pennsylvania. Experimental bullets having the configuration shown in FIG. 2 are described in "History of Modern US Military Small Arms Ammunition," by F. W. Hackley et al, and published by MacMillan and Company in 1967. As a result of the experiments, it was concluded that conventional projectiles with center holes therethrough provided no benefit with respect to air resistance or drag.
Following World War II, considerable information was accumulated concerning internal aerodynamics of supersonic flow in ducts and diffusers for various aerodynamic applications. Much of the accumulated data is useful in the analysis of ballistic projectiles, particularly those of tubular form. Through theoretical study and experimentation, it is known that the normal shock wave generated in front of the air inlet duct of a jet engine of supersonic aircraft as they exceed sonic velocity could be "swallowed" by the duct at some predetermined design Mach number. This refers to a steady-state phenomenon at certain supersonic airflow velocities whereby the normal shock at the duct inlet disappears and mass flow efficiency through the duct rises sharply. The noted phenomenon has application in the design of ballistic projectiles whereby the normal bow shock is not present under ideal supersonic flow conditions, resulting in a dramatic reduction in total drag force. This flow condition requires certain precise combinations with regard to cross sectional size of the internal and external surfaces of the hollow projectile and with further regard to the launching velocity thereof. Since projectiles fired from guns normally receive their total propelling force within the gun tube or barrel, their highest velocity is achieved as they leave the gun muzzle, after which deceleration occurs throughout their projectory. As a result, air flow conditions relating to the external ballistics of any projectile are necessarily transient and never completely constant. However, hollow tubular projectiles can be designed to swallow the normal bow shock at particular launch velocities above Mach 3 and to retain the supersonic internal flow characteristics associated with this phenomenon throughout a certain narrow range of supersonic air flow velocities. It has been found through experimentation that during deceleration of the projectile the internal flow experiences an abrupt change whereby a bow shock wave appears at the nose of the projectile and subsonic flow occurs through the center passage. This condition is called "choking" and is accompanied by a sharp increase in drag.
In recent years, interest has renewed in pursuing the elusive technical answers to ballistical problems familiar to those skilled in the art. Projectile 21 shown in FIG. 3 illustrates one attempt to reduce drag in a hollow projectile by providing a smooth straight cylindrical inner surface 24 of constant cross sectional area throughout the length of the center passage. The external contour of projectile 21 includes a shallow conical or bevel surface 22 forming a leading edge at the inlet of passage 24, while another conical surface 23 intersects with surface 22 and terminates in a trailing edge at the aft end of the projectile. Due to the constant cross sectional area of passage 24 in FIG. 3, supersonic air flow will not occur in the passage. Moreover, although sharp leading and trailing edges are provided by the projectile shape in FIG. 3, use of shallow angles defining conical surfaces 22 and 23 result in very thin wall thickness of the projectile with a consequent low mass. FIG. 4 shows projectile 25 with straight cylindrical outer surface 27 having a constant cross sectional diameter and oppositely directed conical inner surfaces resulting in a throat section at the intersection thereof. Inner surface 26 thus comprises a compression section since the cross sectional area of the inlet at the leading edge of projectile 25 is obviously larger than the cross sectional area at the throat. However, the considerable length of the conical surfaces used for the inner passage of projectile 25 produces the same problem as stated regarding FIG. 3; namely, insufficient projectile mass for use in weaponry. FIG. 5 shows another design approach for projectile 28 wherein the mass is maintained at an acceptable high level through the use of relatively thick walls, while drag is reduced by providing conical surface 30 which intersects surface 29 to provide a sharp leading edge. Actual experiments with projectile 28 have had very disappointing results for reasons which will appear below. The projectile shown in FIG. 6 is a theoretical model for a low drag tubular shape which has been known for years. It suggests a sharp-edged inlet 31 with a gradually tapering inner surface converging toward a throat section 32 immediately followed by a divergent aft section of increasing cross sectional area. While the aerodynamic flow characteristics thus achieved by the shape shown in FIG. 6 provides certain advantages, use of these inner surfaces with a straight cylindrical outer surface results in insufficient mass for military use as a projectile.
In considering the prior art including that represented by FIG. 1 through 6 discussed above, it must be emphasized that projectiles intended for use as weapons are required to have sufficient destructive force upon impact to result in catastrophic or incapacitating damage against hard targets. There is a clear relationship between projectile mass and terminal momentum. Where there is so little mass remaining after shaping the projectile to produce low drag aerodynamic characteristics close to the ideal such as shown in FIG. 6, total loss of usefulness as a destructive device results. Accordingly, conflicting design considerations are presented which involve trade-offs between velocity, mass, aerodynamic characteristics and payload capacity.
The ballistic characteristics of hollow projectiles is a highly empirical science. Minor changes of contour which seem insignificant to the uninitiated can make a decisive difference of success or failure in a design. It is not enough, for example, that the projectile have thin, highly polished walls and sharp leading and trailing edges as in the case of the projectiles shown in FIGS. 1 through 6. It was apparently the designer's objective in FIG. 6 to reduce drag by establishing efficient internal flow through use of the proper ratio of inlet area to throat area. The shallow internal angles shown in FIG. 6 would achieve that objective, but unavoidably produces insufficient projectile mass, which will result in relatively short range for any given launch velocity. The foregoing statement can be illustrated by the following example. If a steel ball-bearing the size of a ping-pong ball leaves a gun barrel with the same velocity used to launch a ping-pong ball, the steel projectile will travel considerably farther than the lightweight projectile due to the difference in their respective mass-momentum characteristics. It is therefore necessary in hollow projectile design that the walls be sufficiently thick to provide reasonable mass to achieve substantial range, and adequate volume to carry a useful payload. However, thick walls inevitably result in higher drag and larger cross-sectional area of the projectile, and it is a principal design objective in this case to overcome these effects aerodynamically.